Is Send Time Optimization (STO) the Real Deal?

Rajat K

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May 28, 2025
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We all know how tough it is to stand out in a cluttered inbox. The idea of emails landing exactly when someone's most likely to open and click sounds like a dream. But is it really?

What's the Big Idea Behind STO?
Think of it like this: Instead of sending your email to everyone on your list at, say, 10 AM on a Tuesday, STO figures out when each individual on your list is most likely to engage. It analyses the historical data on user engagement and sends the email at the right time.

The hype is real because the benefits can be, too:
  • More Opens, More Clicks: The logic is simple: if your email lands when someone is actively checking their inbox, it has a much higher chance of being seen, opened, and clicked. Many marketers report noticeable uplifts here.
  • Personalization, Automated: Imagine trying to manually figure out the best send time for thousands of people! STO automates this, which is a massive win.
  • Cutting Through the Noise: In an overflowing inbox, landing at the "perfect" moment helps your email feel less like spam and more like a welcome message.
So, what are the flaws?
  • Fresh contacts: STO relies on past data. So, what about contacts who are brand new to your list or haven't engaged much yet? The system has no 'personal history' for them.
  • Apple's Privacy Moves (Proxy Opens): If your user an iPhone or Mac, Apple's servers pre-load emails. This means your email might register as "opened" even if the human hasn't actually seen it yet. It inflates open rates and messes with the exact "open time" data STO used to love.
  • Bot Clicks: Similarly, lots of corporate firewalls and security scanners automatically click every link in an email to check for nasties. These are "bot clicks" not a human showing interest, just an automated security check. This inflates click rates and can trick algorithms.
So, is STO still useful with all this "Noise"?
  • STOs send the emails at fallback time (when the email was actually scheduled) when the historical data is not available.
  • Focusing on Real Engagement: Instead of just "opens," STO algorithms are now prioritizing actual human clicks and downstream actions (like visiting your website, making a purchase, or signing up). A bot might "open" an email, but it's rarely going to actually buy something or fill out a form!
  • Smart Filtering: Use of bot detection techniques like IP filtering, behavioral analysis, and machine learning is real and increasingly effective.
The Verdict: It is useful, but it is not a silver bullet!

Yes, STO is “the real deal” when used correctly, especially in larger-scale or enterprise campaigns. But its effectiveness depends on data quality, ESP sophistication, and evolving challenges like privacy shifts and bots. While many ESPs claim these features, quality varies. Some still struggle to filter bot activity accurately.

What are your thoughts? Have you tried STO? Seen any cool results (or frustrating quirks)?
 
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